Current supply apparatus



' June 26, .1962

R. E. KUBA CURRENT SUPPLY APPARATUS Filed Oct'. 15. 1958 .'5 SheebsSheet 1 A TTORNEY CURRENT SUPPLY APPARATUS Filedoct. 15,1958 5 :ssAheets-snee'za` TIME \ /Nl/ENTOR R. E'. KUBA ATTORNEY June 26, 1962 A R E. KUBA 3,041,523'

CURRENT SUPPLY APPARATUS A Filed oct. 15. 195s "i Smeets-sheet s wma-Nro?? l?. E. KUBA Afro/RMV United States Patent O 3,041,523 CURRENT SUPPLY APPARATUS Richard E. Kuba, Columbus, Ohio, assignor to International Business Machines Corporation, New York, N.Y., a corporation of New York Filed Oct. 15, 1958, Ser. No. 767,360 17 Claims. (Cl. 321-16) This invention relates to current supply apparatus and more particularly to apparatus for maintaining a load voltage substantially constant irrespective of amplitude and frequency changes of the voltage source and irrespective of impedance changes of the load.

An object of the invention is to provide improved apparatus for regulating the current supplied from an alternating-current supply source to a load to minimize load voltn age changes.

Another object is to provide improved apparatus for controlling the supply of rectified alternating current to a load to minimize load voltage changes.

Another` object of the invention is to provide improved apparatus for compensating for changes of output voltage of a line voltagereglator due to frequency changes of an alternating-current source for supplying current to the line regulator.

Another object is to provide improved apparatus for compensating for changes of output voltage of a supply of rectified alternating current due to frequency changes of an alternating-current source supplying current to the rectifier.

Another object is to provide improved apparatus for compensating for changes of output voltage of a line voltage regulator due to impedance changes of a load to which current is supplied from the output of the line voltage regulator.

In accordance With the invention, specific embodiments of which are herein described for the purpose of illustration, there is provided for supplying current to a load an alternating-current supply source having a plurality of frequency components including a first and a second frequency component. The second frequency component may be a harmonic of the first or fundamental frequency component, for example.` The frequencies of the frequency components and the resistance or reactance, or both, of the load may change. The alternating current may be supplied, for example, from the output of a line voltage regulator of the permanent type to the input of which is supplied current from a commercial source of alternating current. There is provided a transformer having a primary and a secondary. The transformer may have primary and secondary windings which are insulated from each other, or, if desired, an autotransformer may be employed.

Current is supplied from the supply source or line regulator through the transformer secondary to the load. There is provided a parallel resonant circuit comprising inductance and capacitance in parallel tuned substantially to the fundamental frequency. Current from the supply source or line regulator is supplied to a shunt current path comprising in series, the transformer primary and the parallel resonant circuit. The impedance of the parallel resonant circuit is very high to the fundamental frequency so that the amplitude of the current of fundamental frequency flowing in the shunt current path is very small. However, the parallel resonant circuit has a capacitive reactance at the harmonic frequency. The parallel resonant circuit and the transformer primary form a series resonant circuit tuned to a frequency which may Ibe either somewhat larger than or somewhat smaller than the frequencyy of the second or harmonic frequency component.

The invention will be described in greater detail with reference to the accompanying drawing in which:

"ice

Y FIG. 1 is a schematic view of a current supply circuit embodying the invention;

FIGS. 2 and 3 are schematic views of modifications of the current supply circuit depicted in FIG. l;

FIGS. 4 to 7, inclusive, are diagrams to which reference will be made in explaining the operation of the invention; and

FIG. 8 is a schematic view of a modification of the current supply circuit shown in FIG. 2.

Referring now to the drawing, there is shown in FIG. l a source of nonsinusoidal alternating current 10 for supplying current to a load current 11. There is provided a transformer 12 having a primary winding 13 and a secondary winding 14 wound on a core 15, the winding 14 being connected in series with the source 410 and the load circuit 11. There is provided a parallel resonant circuit comprising an inductance winding 16 wound on a core 17 and a condenser 18. The parallel resonant circuit 16, 18 is tuned substantially to the fundamental frequency of the source 10. A shunt current path connected across the alternating-current source 10 comprises the winding 13 and the parallel resonant circuit 16, 18 in series.

The principle of operation of the invention may be explainedV as follows with the reference to FIGS. 4, 5 and 6. In FIG. 4 there are shown two sinusoidal voltage generators 20 and 21 each having zero internal impedance. The generator 20 produces a Voltage V1 having a fundamental frequency f1, and the generator 21 produces a Voltage V3 havinga frequency f3 which is the third harmonic of the frequency f1. The generators 20 and 21 are connected in series to a load circuit 22 which comprises a bridge rectier 23, a condenser input filter 24 connected to the output of the bridge rectifier and a resistor 25 connected to the output of the filter 24. The filter 24 comprises shunt condensers 26 and 27 and a series inductor 28.

The voltage impressed upon the load circuit is equal to the sum of the voltages V1 and V3. If the amplitude of the fundamental frequency voltage wave V1 were to remain fixed, it will be seen that the Wave shape of the voltage impressed upon the load circuit 22 can be altered by varying the amplitude or the relative phase position, or both, of the third harmonic voltage V3 with respect to the. fundamental frequency voltage V1. Moreover, by

controlling the amplitude and phase of the harmonic frequency voltage relative to the fundamental frequency voltage, the peak voltage, the root-rnean-square voltage and the average voltage which appear at the input of the load circuit 22 are also controlled. Thus the output voltages of load devices, for example, a capacitor input filter which is sensitive to peak input voltages, or load devices sensitive to the rectified average of its input voltage, for example, a choke input filter, or devices sensitive to the root-mean-square input voltage, such as resistive loads, may be controlled.

Of course, a voltage having a frequency other than the third harmonic frequency could be used instead of thev third harmonic frequency voltage, in accordance with the invention. present simultaneously and still provide effective control.

- Sub-harmonic voltage components could also be used.

The invention may be used to control the current supplied to a load from an alternating-current supply source of load current changes and irrespective ofA changes of load power factor.

Referring again to FIG. 1 of the drawing, the nonsinus'! oidalV alternating-current source 10 may be a ferro-reso nant type line voltage regulator to which current is supplied from a commercial source of alternating current. It

In fact, several or all harmonics could be .y

is well known that this type of voltage regulator generally exhibits a highly nonsinusoidal wave form at its output terminals. The transformer core 15 may contain an air gap which may lbe a single `gap or a stepped gap or the core may be partially gapped and partially interleaved or the gap may be entirely interleaved, depending upon the amount and nature of the compensation required. 'The load circuit 11 may be a bridge rectifier connected to a capacitor input filter and having an output load resistance, as shown for the load circuit 22 of FIG. 4. Alternatively, the load circuit 11 could be a rectifier connected to a choke input filter and output load resistance. The core 17 has a gap which may be a single air gap or a stepped air gap, or the core may be partially gapped and partially interleaved, depending upon the nature and amount of compensation desired.

The inductor comprising winding 16 and core 17 with its air -gap are so designed that the self-inductance of winding 16 will resonate with the capacitance of condenser 18 at approximately the fundamental frequency of the voltage source 10. If f1 is the frequency of the fundamental frequency component of the source 10, C is the capacitance of condenser 18 and L is the inductance of winding 16, then,

Under steady state conditions of operation, the circuit of FIG. 1 can be represented for the high nth harmonic frequency component of the voltage source 10 of FIG. l by the approximately equivalent electrical circuit shown in FIG. 5. The circuit of FIG. 5 is like the circuit of FIG. l except that there is provided a condenser 30' having an equivalent capacitance Cn defined by Equation 2, in place of the parallel resonant circuit 16, 18 of FIG. 1, and that the voltage is the equivalent sine Wave voltage for the nth harmonic of the fundamental voltage of the source 10 of FIG. 1. This equivalent circuit neglects all resistances associated with the circuit elements of FIG. 1.

The inductance of Winding 13 (FIG. 5), is preferably designed so that the series resonant frequency of the shunt path comprising the self-inductance of winding 13 and the capacitance Cn lies either somewhat above or somewhat below the frequency of the predominant harmonic voltage component of the source 10 of FIG. 1. In many cases the predominant harmonic is the third harmonic. If nl is the number of the harmonic of highest amplitude, L13 is the self-inductance of winding 113 (FIG. 5), Cm is the equivalent capacitance of the capacitor 30 for the predominant harmonic frequency of f1 is the fundamental frequency of the alternating-current power source, then L13 and Cm are designed to have values of inductance and capacitance such that f 21W [1130111 The choice of value for L13 and Cm depends on the type of compensation desired. The turns ratio of transformer windings 13 and 14 is selected to introduce just the correct amount of compensating voltage at the terminals of winding 14 such that the desired compensation is achieved.

Since the parallel circuit is tuned to approximately the fundamentl frequency of the voltage source 10, this parallel resonant circuit offers a high impedance to the flow of current of fundamental frequency. Therefore, the fundamental frequency voltage component impressed upon the load circuit 11 (L) will be related to the fundamental frequency voltage component of the source 10 (s) by the complex vector equation L=S21F1L14L (5) where f1 is the fundamental frequency of the input voltage L is the complex vector fundamental component of load current and L14 is the self-inductance of winding 14. By making the inductance of the transformer secondary 14 very small such that, even for large ranges of values of the fundamental component of load current,

then, obviously,

For this condition, the fundamental component of the voltage of the source 10 is substantially unaffected by the componsating circuit comprising transformer 13, 14 and the parallel tuned circuit 16, 18` and it therefore appears at the terminals of the load circuit 11 substantially unaltered.

However, the harmonic components of the voltage of source 10 are appreciably affected by the compensating circuit because the parallel tuned circuit 16, 18 behaves as an equivalent cpacitance Cn at the harmonic frequencies which, therefore, can present a low impedance to the flow of harmonic currents. All of the harmonic cornponents of the voltage source 10 will be affected in some degree by the compensating circuit, ybut usually it is the predominant harmonic component or the two harmonics lhaving amplitudes larger than the remaining harmonics which are of greatest importance in controlling the output or load voltage.

If the tuning of the equivalent capacitance Cn (FIG. 5) and inductance L13 is such that 1 1 27a/Laon then transformer winding 14 sees an equivalent capacitance and, if

then transformer winding 14 sees an equivalent inductance.. Harmonic load current flowing through the equiv alent capacitance, under the condition 1 'L1f1 21a/Lac..

for example, produces a phase shift and change in ampliC tude of the harmonic component of voltage appearing at the terminals of the load circuit 11 relative to the fundamental component of voltage appearing at the terminals of the load circuit. The wave shape of the voltage impressed upon the load circuit 11 thus varies as a` function of load current.

The amount of amplitude variation and the amount of phase shift variation is dependent upon the tuning of the series combination of the inductance L13 and the capacitance Cm. Therefore, if the frequency of the source voltage is changed, the compensating circuit 13, 14, 16, 18 will be responsive to the frequency change to control the voltage and shape of the wave impressed upon the load circuit.

Moreover, if the inductance of winding V13 is of the swinging type achieved by step gapping or partially interleaving core 15, then the inductance of winding 13 will be a function of load current and a nonlinear type of compensation can be obtained. This effect is very useful in compensating regulators in which load voltage changes produced in response to load changes, are larger at light loads than at heavy loads.

.By making the inductance of winding 16 of the swinging type by employing a core 17 having a stepped gap or a partial gap and partial interleaving of the core laminations, the tuning of the parallel tuned circuit 16, 18 can be made less susceptible to changes of input frequency, thus resulting in a more favorable frequency compensation effect.

The phase angle ofthe harmonic voltage appearing across winding 14 is `dependent upon the phase angle of the harmonic current flowing through the winding 14. But the phase angle of the harmonic load current is determined primarily by the power factor of the load. The compensating circuit 13, 14, 16, 18 is thus sensitive to the load power factor.

A mathematical analysis of the circuit of FIG. l shows more clear-ly the functional dependence of the load voltage and its wave shape on load current, load power factor, input frequency and inductive nonlinearities. The circuit of FIG. 6 to which reference will now be made is like FIG. l but the tuned circuit 16, 18 has been replaced by a lumped, complex, linear impedance Z.

The following quantities are defined as indicated below:

m-complex, vector input voltage, sine wave lilo-complex, vector output Voltage, sine wave ltr-complex, vector output current, sine wave Nl-number of turns of winding 14 N2-number of turns of winding 13 Ll--self-inductance of Winding 14 Lz-self-inductance of winding 13 f-frequency Z-complex, vector impedance at frequency j The following assumptions are made:

(l) All circuit resistances are neglected.

(2) Transformer core 15 is gapped and the transformer 13, 14 is treated as a linear device.

(3) Steady state, sinusoidal conditions prevail.

The output voltage D is related to the input voltage Em by the equation,

2 (fi) Le@ If |wL10| |mL then O, that is, the fundamental component of voltage is not appreciably disturbed byl the compensating circuit.

Let us next consider Equation 8 where the -freqency of the input voltage is a harmonic of the fundamental frequency. The impedance Z is a purely capacitive impedance so that since where C:n is defined in Equation 2.

Substituting A for Z in Equation 8 and simplifying,

Equation l0, which applies to the harmonic components only, fully supports the following conclusions:

(l) The amplitude and phase position of the harmonic component voltages relative to the fundamental frequency component are a function of the harmonic load currents, the frequency of the alternating-current source, the power .factor of the load, the turns ratio and self-inductance of transformer Windings 13 and y14, and the equivalent capacitance of the parallel combination of inductor 16 and capacitor 18.

(2) Additional compensation can be obtained by allowing the inductances L1 and L2 of windings 14 and 13, respectively, to be a function of the load current, that is, by employing inductances L1 and L2 of the swinging typel obtained through the use of stepped gaps in the transformer core 15.

As indicated in FIG. 1 by the dots adjacent to the windings 13 and 14, these windings are Wound in opposite directions starting at a common terminal connected to an output terminal of the voltage source 10. These windings, instead of being Wound oppositely, as shown, may be wound in the same direction, that is, one of the windings 13 and 14 may be reversed. In this modified arrangement, the range of control is extended because, although the fundamental frequency voltage component is nearly unaffected by this polarity reversal, the magnitude and phase of the harmonic frequency voltage c0mponents are affected and can be modified over a greater range of variation. The equations applying to this moditied embodiment of the invention are as follows:

for the fundamental frequency voltage component. However, Equation ll yields +3 (arnfnLor 1 (13) for the nth harmonic voltage. A comparison of Equations 13 and l0 shows that the harmonic voltage can be controlled over extended ranges when reversing one of the transformer windings 13 and 14 of FIG. 1.

Referring now to the embodiment of the invention shown in FIG. 2, there is provided a ferroresonant voltage regulator 31 the core structure of which comprises the outer 0-shaped laminations 32 and the inner T-shaped laminations 33. Windings 34, 35 and 36 are placed in position upon the stack-up of the T-shaped laminations 33, as shown, and the assembly of laminations 33 and windings 34, 35 and 36 is then pressed into position inside the similar stack-up of the O-shaped laminations 32. Air gaps 37 and 38 are in a shunt magnetic flux path. Winding 14 is connected to a source of commercial alternating current 39 which, for example, may be a 11S-volt, n50-cycle per second source. A condenser 4l)` is connected across windings 35 and 36 in series. The combination of windings 35 and 36 and condenser 40 produces a ferroresonant condition of operation in the regulator 31. The output voltage -across winding 35 ofthe line voltage regulator 31 is a nonsinusoidal voltage typically shown in FIG. 7. A harmonic analysis of this wave form at full load condition indicates the following harmonic maximum voltage expressed as a percentage of the fundamental maximum voltage; third harmonic, 19.7 percent; fifth harmonic, 8.9 percent, and seventh harmonic, 3.4 percent.

There is provided a transformer 41 having a core comprising a 11/2-inch stack-up of El laminations 43 and 44 of 29-gauge transformer steel, butt jointed with an air gap 47 of `0.020 inch for 11A-inch of the stack-up and butt jointed with an air gap of 0.003 inch for 1/4 inch of the stack-up. A primary winding 45 and a secondary winding 46 are wound on the middle leg of the threelegged core formed by the laminations 43 and 44. Winding 45 has 248 turns of No. 17 wire and winding 46 has 21 turns of No. 16 wire. The inductance of winding 45 varies from 0.248 henry at a 60-cycle per second current of 0.107 `ampere t 0.199 henry at a 60-cycle per second current of 1.33 amperes.

There is provided a reactor 51 having a core 'comprising a one inch stack-up of 29-gauge EI laminations 53 and 54 of grain oriented steel butt jointed with an air gap 57 of 0.017 inch. A Winding 55 on the middle leg of the core has 870 turns of No. 25 wire. The inductance of reactor 51 is 0.828 henry and its Q is equal to 9.33 at a frequency of 60 cycles per second and an applied sinusoidal voltage of 100 volts root-mean-square, Q being the ratio of inductive reactance to resistance. A condenser 58 having a capacitance of 8.23 microfarads for a frequency of 60 cycles per second is connected across the winding 55. The parallel combination of winding 55 and condenser 58 is connected in series with transformer winding 45 across the winding 35 of the ferroresonant line voltage regulator 31.

Current is supplied from the winding 35 through the transformer secondary winding 46 to the input of a bridge rectifier 60 employing germanium diodes and having positive and negative output terminals. The rectifier output is connected through a condenser input filter 611 to a resistive load 62. The filter comprises shunt condensers 63 and 64 of 20,000 microfarads and 15,000 microfarads, respectively, and a series inductor 65. "Ehe inductor 65 comprises a 11/8 inch stack-up of EI, 29- Ygauge transformer steel laminations 66 and 67 butt jointed with an air gap 68 of 0.039 inch. Winding 69 on the middle leg of the three-legged core has 56 turns of No. square Wire. The inductance of the filter choke 65 measured 0.00285 henry. The root-meansquare ripple voltage at the output of the filter 63, 64, 65 was 0.031 volt at a direct load current of 6.5 amperes.

The resonant frequency of the parallel combination of capacitor 58 and inductance winding 55 is calculated Where L is the inductance of winding 55 in henrys, C is the capacitance of condenser 58 in farads and Q is the quality factor of winding 55 at 60 cycles per second.

The series resonant frequency of winding 45 in series with the parallel combination of capacitor 58 and winding 55 is given by the equation cycles per second of the line voltage source 39.` The series resonant frequency as determined from Equation 15 varies from 127.48 cycles per second to 138.08 cycles per second as the inductance of winding 45 swings from 0.248 henry to 0.199 henry with increasing load.

The circuit of FIG. 2 was tested with and without the compensating circuit comprising transformer 41, inductor 51 yand condenser 58. With the compensating circuit, the voltages across the load 62 were found to be 58.3, 58.1, 58.1, 58.1 and 58.0 volts for load currents of zero, 2.20, 3.85, 5.40 and 6.50 amperes, respectively. Without the compensating circuit the load voltages were found to lbe 60.0, 58.4, 57.7, 57.2 and 56.8 volts for lload currents of zero, 2.19, 3.80, 5.48 and 6.64 amperes, respectively. These tests were performed with a line voltage of `source 39 of 115 volts at a frequency of 60 cycles per second which was held constant. Similar data are obtained when the tests are repeated at line voltages as much as ten percent above or below the -volt input voltage.

A modification of the current supply circuit of FIG. 2 is shown in FIG. 3, similar components being identified in the two figures by the same numerals. In FIG. 3, the ferroresonant line voltage regulator comprises, in addition to the winding 34, the windings 70, 71 and 72 across which the condenser 40 is connected. The transformer 41 comprises windings 73, 74 and 75. 'Ilhe rectifier comprises rectifying elements 76 and 77. The transformer windings are each ywound in the same direction starting from a start terminal S to a finish terminal F as shown in the drawing. Current is supplied from windings 70 and 71 of the ferroresonant regulator 31 to a current path comprising winding 74 and the parallel resonant circuit 58, 55 in series, the parallel resonant circuit being tuned substantially to the fundamental frequency of the source 39. The circuit comprising winding 74 in series with the parallel resonant circiut 58, 55 is tuned to a frequency somewhat lar-ger than or somewhat less than the frequency of a harmonic of the fundamental frequency of the source 39.

Current is also supplied alternately through a circuit which may be traced from the upper terminal of winding 70 through winding 73, rectifying element 76, inductor 69 and load 62 to the common terminal of windings 70 and 71 and through a circuit which may be traced from the common terminal of windings 711 and 72 through winding 75, rectifying element 77, inductor 69 and load 62 to the common terminal of windings 70 and 71. The transformer 41 employs two secondary windings 73 and 75 to provide a balanced compensating effect.

If desired, more than one output can be provided from a single ferroresonant regulator like the regulator 31 of FIG. 2, for example. In this case, a current supply circuit, like the current supply circuit 41, 51, 58, 60 and 65 of FIG. 2, for example, could be provided for each of the output circuits.

Moreover, the principle of the invention would apply to regulators energized from polyphase sources of alternating current, as in the embodiment of the invention shown in FIG. 8, for example.

FIG. 8 depicts a modification of the inveniton shown in FIG. 2 adapted for three-phase operation. The components of FIG. 8 are designated by the numerals used in FIG. 2 for identifying corresponding components except that, where three similar components are required in FIG. 8, because of the three-phase operation, each numeral is followed :by the letter a, b or c.

Referring now to FIG. 8, there are provided three ferroresonant voltage regulators 31a, 3117 and 31e, each like the regulator 31 of FIG. 2. The windings 34a, 34b and 34C of the ferroresonant regulators, respectively, are connected in la delta configuration to the phases 39a, 39b and 39C, respectively, of a three-phase commercial source of alternating current. The windings 35a', 356 and 35C are connected in la star configuration, that is, these windings have a common terminal. Condensers 40a, 40b and 40C are connected, respectively, across windings 35a and 36a, windings 35b 'and 36b, and windings 35e and 36C. There is `connected across the Winding 35a a shunt current path comprising winding '45a and the parallel tuned circuit 58a, 55a like the shunt current path 45, 58, 55 of FIG. 2. 'l'lhe current path-s `4511, SSb, 55b and 45C, 58e, 55e are similarly connected across windings 35b and 35e, respectively.

The terminal of w-inding 35a other than the common terminal is connected to a firs-t terminal of winding 46a anda second terminal of winding 46a is connected to one of the three input terminals of a three-phase bridge rectiier 60. Terminals of windings 35b and 35C are similarly connected through windings 4Gb and 46c to other input terminals of rectifier 60, respectively. The output terminals of rectifier 60 are connected through a condenser input lter 65 to a load 62.

What is claimed is:

l. Apparatus for supplying current from an alternatingcurrent supply source to a load comprising a vol-tage regul'ator having an input connected to said supply source, said regulator .being of a type which distorts the yalternating wave impressed upon its input and thereby introduces into the wave supplied from its output at least one frequency component `of considerable amplitude which is a harmonic of the fundamental frequency of said alternating Wave, a transformer having a first and a second winding, means for supplying current from the output of said regulator to a iirst circuit comprising in series said first transformer Winding and a load, a tuned circuit compris ing inductance and capacitance in parallel tuned substantially to the fundamental frequency of said alternating wave, means for supplying current from the output tof said regulator to a second -circuit comprising in series said second transformer Winding `and said tuned circuit, said second circuit being tuned to la frequency in the vicinity of but differ-ing from the frequency of said harmonic.

2. Apparatus for supplying rectified current from an alternating-current supply source having a fundamental frequency to a load comprising a voltage regulator having an input connected to said supply source and yan output, said regulator distorting the alternating wave impressed upon its input to `cause to be supplied from its output fa Wave having a component of said fundamental frequency and at least one frequency component of considerable :amplitude which is a harmonic of said fundamental frequency, a transformer having a primary and `a secondary winding, means for supplying current from the output of saidregulator through said transformer secondary to 1a load, a capacitor, an inductor, means for connecting said inductor and capacitor in parallel to form a tuned circuit which is resonant substantially to said fundamental frequency, Iand means for supplying current from th'e outputof said regulator to a current path cornprising in series said primary transformer winding and said tuned circuit, said current path having a resonant frequency which is higher than said fundamental frequency land which differs from said harmonic frequency.

3. Apparatus for supplying current from an alternatingcurrent supply source having a fundamental frequency to a load comprising a voltage requlator having an input and 'an output, a transformer having a primary and a secondary, means for connecting the input of said regulator to said supply source, means for supplying current from the output of said regulator through said secondary to said load, the output voltage of said regulator having a component of said fundamental frequency and 'at least one frequency component of considerable amplitude which is la harmonic of said fundamental frequency, impedance means, and a shunt current path across the output of said regulator comprising in series said primary and said impedance means, the impedance of said impedance means being suciently high to current of said `frequency different than the frequency of said first component, a parallel resonant circuit tuned substantiallyA to the frequency of said first component, a first current path comprising said secondary and a load in series, a second current path comprising in series said primary and said parallel resonant circuit, fand means for connecting said irst and second current paths in parallel across said alternating-current source, said second current path forming a series resonant circuit tuned to a yfrequency other than the frequencies of said first land second components.

5. A combination in accordance with claim 4 in which the number of turns of said primary is several times at least the number of turns of said secondary.

6. A combination in accordance with claim 5 in which said transformer comprises a core forming a rst and a second flux path of magnetic material, one at least of said flux paths having an air gap therein.

7. A combination in accordance with claim 4 in which said p-arallel resonant circuit comprises a capacitor and an inductor connected in parallel, said inductor comprising a winding on a core, said core for-ming a rst and a second flux path of magnetic material, one at least of said flux paths having an air gap therein.

8. Apparatus for supplying current from an alternating-current supply source to a load circuit comprising voltage regulating means having a pair of input terminals connected to said supply source and a pair of output terminals for reducing voltage changes across said output terminals with respect to voltage changes of said supply source, the voltage across said output terminals having a fundamental frequency component and a component which is a harmonic of said fundamental frequency, and means for supplying current from said output terminals to said load circuit comprising means for controlling the current of said harmonic frequency component with respect to the current of said fundamental frequency component for reducing voltage changes across said load circuit with respect to Voltage changes across said output terminals.

9. Apparatus in accordance with claim 8 in which said means for supplying current from said output terminals of said voltage regulating means to said load circuit comprises means for controlling the amplitude and phase of the current of said harmonic frequency component with respect to the amplitude and phase respectively of the current of said fundamental frequency component.

10. In combination, a ferroresonant line voltage regulator having input and output terminals, means 4for connecting said input terminals to a source of alternating current, the voltage across said output terminals having a voltage component of fundamental frequency and a voltage component of a harmonic lfrequency, a transformer comprising a primary and a secondary winding wound on a core of magnetic material having a relatively large air gap in one cross-sectional portion of the core and a relatively small air gap in another cross-sectional portion of the core, the number of turns of said primary winding being several times at least the number of turns of said secondary Winding, a reactor comprising a winding on a core of magnetic material having an .air gap therein, a condenser connected across said reactor winding, said reactor winding and condenser in parallel having a resonant frequency substantially equal to said fundamental frequency, means for connecting said parallel connected reactor winding and condenser in series with said primary winding to said output terminals to form a series 1 1 resonant circuit tuned to a frequency greater than said fundamental frequency and less than said harmonic frequency, and means for supplying current from said output terminals through said secondary winding to a load circuit.

11. In combination, a ferroresonant `line voltage regulator comprising primary and secondary windings Wound on a core of magnetic material, means for connecting said primary to a source of `alternating current having a fundamental frequency, a condenser connected across said secondary Winding, said secondary winding having a first terminal, a second terminal, and a third terminal intermediate said first and second terminals, a transformer having a first, a second and a third winding, a first and a second rectifying element, a ripple filter, having a pair of input terminals, a first current path comprising in series said first winding and said first rectifying element for connecting said first terminal to one of said filter input terminals, a second current path comprising in series said second winding and said second rectifying element for connecting said second terminal to said one filter input terminal, a parallel resonant circuit tuned to said fundamental frequency comprising a reactor and a condenser connected in parallel, a third current path connected to said first and second terminals comprising in series said parallel resonant circuit and said third transformer winding, said third current path being tuned to a frequency several times said fundamental frequency, means for connecting said third terminal to the other of said filter input terminals, and means for supplying current from the output of said filter to a load.

12. In combination, three ferroresonant line voltage regulators each having a primary winding and a secondary Winding Wound on a core of magnetic material, means for connecting said primary windings in delta to a source of three-phase alternating current, each of said secondary windings having a first and a second terminal, means for conductively connecting said first terminals, three transformers each having a primary and a secondary, three parallel resonant circuits each tuned substantially to the frequency of said alternating-current source, shunt current paths connected across said regulator secondary windings comprising in series said transformer primaries and said parallel tuned circuits respectively, each of said shunt paths being series resonant to a frequency substantially higher than the frequency of said alternatingcurrent source, and. means for connecting the second terminals of said regulator secondary windings through said transformer secondaries respectively to a three-phase load.

13. The combination with a -source of alternating current having a first and a second frequency component of a transformer having a primary and a secondary winding, a first and a second current path, means for connecting said current paths in parallel across said current source,`

said first current path comprising said secondary winding and a load in series, said second current path comprising said primary winding, and means in said second current path for suppressingv said fi-rst frequency component and for transmitting said second frequency component, the inductance of said primary winding decreasing in response to increasing load;

14. Apparatus for supplying to a load current from an alternating-current supply source having a component of fundamental frequency and a component which is a harmonic of said fundamental frequency comprising a transformer having a primary and a secondary, a parallel resonant circuit tuned substantially to said fundamental frequency, means for impressing across said load a voltage component substantially equal to the voltage of said fundamental frequency component comprising means for connecting said secondary in series with said supply source and said load, and means for introducing into said load circuit a voltage of said harmonic frequency which varies lin response to load current changes to minimize voltage changes across said load, said means cornprising means for connecting said primary in series with said parallel resonant circuit land said supply source.

15. In combination, a transformer having a primary and a secondary, a source of alternating current having a fundamental frequency component and a harmonic frequency component, means for supplying current from said source to a first circuit comprising said secondary only of said transformer and a load in series, a parallel resonant circuit tuned substantially to said fundamental frequency, and means for supplying current from said source to a second circuit comprising in series said primary and said parallel resonant circuit, said second circuit forrning a series resonant circuit tuned to a frequency other than said fundamental and harmonic frequencies.

16. A combination in accordance with claim 15 in which the tuning o-f said series -resonant circuit varies in response to load current changes.

17. The combination with `a source of alternating current having a first frequency component and a second frequency component of a transformer having a primary and a secondary winding, a parallel resonant circuit tuned substantially to the frequency of said first component, a first and a lsecond current path, means for connecting said current paths in parallel across saidcur-rent source, said first current path comprising said secondary winding only of said transformer and a load in ser-ies, said second current path comprising said primary winding only of said transformer and said parallel resonant circuit in series for suppressing said first frequency component and for transmitting said second frequency component, the inductance of said primary winding decreasing in response to increasing load.

References Cited in the file of this patent UNITED STATES PATENTS 1,878,350 Thompson Sept. 20, 1932 2,052,338 Budenbom Aug. 25, 1936 2,442,214 Short May 25, 1948 -2,505,620 John et al. Apr. 25, 1950 2,804,588 Hjermstad Aug. 27, 1957 

